"The acting Attorney General, Sally Yates, has betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States," the White House press secretary's office said in a statement.

What is wrong with Trump's immigration ban?

Yates, an Obama appointee, had agreed to serve in an acting capacity until Senator Jeff Sessions, Trump's pick, is confirmed by the Senate.

The Yates decision came as Trump pressed into his second week in office defending his sweeping immigration ban.

Trump's order temporarily suspends the US refugee programme and bars all immigration for citizens of Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen for at least 90 days.

Reaction to the order was swift, with protests erupting at major airports and other major cities including the capital Washington DC.

Trump denied that his order was to blame for weekend chaos at the nation's airports, instead pointing to computer glitches, demonstrations and even the "fake tears" of a senior opposition senator, Democrat Charles Schumer.